The transportation of items of freight by air is increasingly gaining in importance, in particular by reason of the very short transportation-times that are possible in this case. However, many items of freight also have to be cooled during transportation, in which connection an interruption of the cold-chain is in many cases not desired or permitted. In order to guarantee appropriate storage conditions for items of freight to be subjected to cold storage during transportation on board an aircraft, two different approaches are pursued at the present time.
A first approach consists in air-conditioning a freight compartment or freight-compartment zone, in which items of air freight to be subjected to cold storage are accommodated, with the aid of the air-conditioning unit of the aircraft. To this end, the freight compartment or freight-compartment zone to be cooled is connected to the air-conditioning unit of the aircraft via additional ports which are provided on the air-conditioning unit of the aircraft. Under certain conditions—for example, the presence of an appropriate ambient or external temperature, a stable cabin temperature and the presence of an additional floor insulation of the freight compartment—and also on the assumption that the cabin doors and freight-compartment doors are closed, temperatures within the range between +5° C. and +25° C. in the freight compartment or freight-compartment zone to be cooled can be adjusted with the aid of the air-conditioning unit of the aircraft.
In principle, an air-conditioning unit of an aircraft is provided with two independent climate-control systems. A first climate-control system of the air-conditioning unit of the aircraft serves for air-conditioning the aircraft cabin, whereas a second climate-control system of the air-conditioning unit of the aircraft can be optionally employed either for the purpose of assisting the first climate-control system for air-conditioning the aircraft cabin or for the purpose of air-conditioning the freight compartment or freight-compartment zone to be cooled. Since a higher priority is granted to the provisioning of the cabin than to the cooling of the freight compartment, the second climate-control system which is provided for the purpose of air-conditioning the freight compartment to be cooled is automatically utilised for the purpose of assisting the first climate-control system for air-conditioning the aircraft cabin if the cabin has a heightened cooling requirement.
An aircraft air-conditioning unit equipped with two independent climate-control systems is relatively complex and expensive. Furthermore, an aircraft air-conditioning unit of such a type has an increased weight and also an increased consumption of energy. Lastly, in the event of a heightened cooling requirement of the cabin, adequate air conditioning of the freight compartment can no longer be ensured in some circumstances, since the second climate-control system which is actually provided for the purpose of air-conditioning the freight compartment is automatically called upon for the purpose of assisting the first climate-control system for air-conditioning the aircraft cabin. A further disadvantage of the cooling of the freight compartment or freight-compartment zone with the aid of the air-conditioning unit of the aircraft consists in the fact that the air-conditioning unit of the aircraft has only a limited cooling capacity. Lastly, in the case of the cooling of the freight compartment or freight-compartment zone with the aid of the air-conditioning unit of the aircraft the entire freight compartment, or at least a relatively large freight-compartment zone, is cooled, so that items of freight that have to be transported at relatively high temperatures cannot be loaded into the same freight compartment or freight-compartment zone. As a consequence, in some circumstances the freight-transport capacity of the aircraft cannot be utilised optimally.
With a view to better utilisation of the freight-transport capacity of an aircraft it is therefore known to employ—in addition to, or as an alternative to, a cooling of the freight compartment or freight-compartment zone with the aid of the air-conditioning unit of the aircraft—cooled special containers for the transportation of items of freight to be subjected to cold storage. Special containers of such a type include a storage tank for receiving dry ice, and also a battery-operated fan. The fan serves, firstly, to convey air over the dry ice acting by way of heat-sink and, subsequently, to blow the cooled air over the items of freight to be cooled. The operation of the fan is controlled ordinarily with the aid of an electronic control unit. Depending on the load-state of the dry-ice storage tank, temperatures within the range between −20° C. and +20° C. can be adjusted in a cooled special container. An extension of this temperature range to a range between −20° C. and +30° C. is possible through the integration of an additional heating unit into the special container. However, the lowest temperature of −20° C. is attained in this case only in the unstable state—i.e. a constant cooling at −20° C. is not possible.
However, special containers of such a type, which are cooled with the aid of dry ice, exhibit the disadvantage that an elaborate consumption calculation—which is dependent on various factors such as, for example, the environmental conditions, the useful load and also the target temperature—for determining the quantity of dry ice to be additionally loaded into the storage tank is required prior to loading. Furthermore, by virtue of the requirement of having to add to the load up to 300 kg of dry ice, and also by reason of the weight of the further components that are required for the cooling of the container, the special container exhibits a higher empty weight and consequently a diminished loading capacity. Moreover, the service life is limited by reason of the limited storage capacity in respect of dry ice and also by reason of the limited battery capacity of the fan.
Further disadvantages arise directly through the use of dry ice by way of heat-sink. For example, by reason of the fact that no fully uniform distribution of the air over the items of freight is possible with the fan that is employed for distributing the cooled air, the cooling with dry ice results in an inhomogeneous temperature distribution in the container. Furthermore, dry ice releases CO2 and therefore has to be declared as a hazardous material. As a consequence, regulations relating to the transportation of hazardous materials and also maximal loadings, which are ordinarily dependent on the type of aircraft, have to be adhered to in connection with the transportation of dry ice. Lastly, the transportation of certain items of freight together with dry ice is prohibited, or at least minimum distances between the items of freight and the dry ice have to be adhered to.
These approaches and/or other approaches are reflected in some of the prior art. For instance, DE 199 52 524 A1 discloses a device for supplying cooling air to a plurality of trolleys 7 disposed in a galley 1 on board a passenger aircraft. The device comprises an air chiller 3 connected to the galley 1 via a cooling circuit 15 so as to supply air cooled in the air chiller 3 to the trolleys 7 disposed in the galley 1.
EP 0 655 593 A1 describes a cooling system 1 for cooling food stored in trolleys 8a, 8b which are disposed in galleys 3a to 3e on board an aircraft. The system 1 comprises a refrigerating device 4 which supplies cooling fluid to heat exchangers 9a, 9b; 17a, 17b. The heat exchangers 9a, 9b; 17a, 17b serve to transfer cooling energy generated by the refrigerating device 4 to cooling air circulating in cooling air circuits 12a, 12b so as to cool the trolleys 8a, 8b in the galleys 3a to 3e. 
US 2003/0042361 A1 relates to a system for cooling a galley on board an aircraft. The system comprises an air chiller 30 which is connected to a heat exchanger 26 disposed in a galley plenum 22 via an intermediate working fluid circuit 27. Lines 25 connect the galley plenum 22 with a galley cart 20 disposed in the galley. Cooling air which receives cooling energy from the heat exchanger 26 disposed in the galley plenum 22 is circulated from the galley plenum 22 to the galley cart 20 via the lines 25 by means of a blower 24.
U.S. Pat. No. 3,507,322 relates to an apparatus for handling perishable materials under controlled temperature conditions in storage and shipment. The apparatus comprises a primary heat exchanger 8 disposed in a primary heat exchanger circuit 7. The primary heat exchanger circuit 7 is in thermal contact with heat conductive piping, tubing or coil circuits 6 embedded in insulated casing 2 of portable containers 1 for storing the perishable materials. A liquid heat transfer medium as circulated through the piping, tubing or coil circuits 6 for transferring heat from the containers 1 to the primary heat exchanger circuit 7.
The object underlying the invention is to make available a cooling system and also a freight container capable of being connected to the cooling system, with which an energy-efficient and reliable cooling of items of freight on board an aircraft is possible.